Perl

1
Perl

• Major parts of this lecture adapted from
  http//www.scs.leeds.ac.uk/Perl/start.html

2
Why Perl?

• Perl is built around regular expressions
• REs are good for string processing
• Therefore Perl is a good scripting language
• Perl is especially popular for CGI scripts
• Perl makes full use of the power of UNIX
• Short Perl programs can be very short
• "Perl is designed to make the easy jobs easy,
  without making the difficult jobs impossible." --
  Larry Wall, Programming Perl

3
Why not Perl?

• Perl is very UNIX-oriented
• Perl is available on other platforms...
• ...but isn't always fully implemented there
• However, Perl is often the best way to get some
  UNIX capabilities on less capable platforms
• Perl does not scale well to large programs
• Weak subroutines, heavy use of global variables
• Perl's syntax is not particularly appealing

4
What is a scripting language?

• Operating systems can do many things
• copy, move, create, delete, compare files
• execute programs, including compilers
• schedule activities, monitor processes, etc.
• A command-line interface gives you access to
  these functions, but only one at a time
• A scripting language is a "wrapper" language that
  integrates OS functions

5
Major scripting languages

• UNIX has sh, Perl
• Macintosh has AppleScript, Frontier
• Windows has no major scripting languages
• probably due to the weaknesses of DOS
• Generic scripting languages include
• Perl (most popular)
• Tcl (easiest for beginners)
• Python (new, Java-like, best for large programs)

6
Perl Example 1
!/usr/local/bin/perl Program to do the
obvious print 'Hello world.' Print a
message
7
Comments on Hello, World

• Comments are to end of line
• Perl statements end with semicolons
• Perl is case-sensitive
• Perl is compiled and run in a single operation

8
Perl Example 2
!/ex2/usr/bin/perl Remove blank lines from a
file Usage singlespace lt oldfile gt
newfile while (line ltSTDINgt) if (line
eq "\n") next print "line"
9
More Perl notes

• Scalar variables start with
• Scalar variables hold strings or numbers, and
  they are interchangeable
• Examples
• priority 9
• priority '9'
• Array variables start with _at_

10
Perl Example 3
!/usr/local/bin/perl Usage fixm ltfilenamesgt
Replace \r with \n -- replaces input
files foreach file (_at_ARGV) print
"Processing file\n" if (-e "fixm_temp")
die " File fixm_temp already exists!\n"
if (! -e file) die " No such file
file!\n" open DOIT, " tr \'\\015'
\'\\012' lt file gt fixm_temp" or die
" Can't tr '\015' '\012' lt infile gt
outfile\n" close DOIT open DOIT, " mv
-f fixm_temp file" or die " Can't mv -f
fixm_temp file\n" close DOIT
11
Arithmetic in Perl
a 1 2 Add 1 and 2 and store in a a
3 - 4 Subtract 4 from 3 and store in
a a 5 6 Multiply 5 and 6 a 7 /
8 Divide 7 by 8 to give 0.875 a 9
10 Nine to the power of 10, that is, 910 a
5 2 Remainder of 5 divided by 2 a
Increment a and then return
it a Return a and then
increment it --a Decrement a
and then return it a-- Return a
and then decrement it
12
String and assignment operators
a b . c Concatenate b and c a b x
c b repeated c times a b
Assign b to a a b Add b to a a
- b Subtract b from a a . b
Append b onto a
13
Single and double quotes

• a 'apples'
• b 'bananas'
• print a . ' and ' . b
• prints apples and bananas
• print 'a and b'
• prints a and b
• print "a and b"
• prints apples and bananas

14
Arrays

• _at_food ("apples", "bananas", "cherries")
• But
• print food1
• prints "bananas"
• _at_morefood ("meat", _at_food)
• _at_morefood ("meat", "apples", "bananas",
  "cherries")
• (a, b, c) (5, 10, 20)

15
push and pop

• push adds one or more things to the end of a list
• push (_at_food, "eggs", "bread")
• push returns the new length of the list
• pop removes and returns the last element
• sandwich pop(_at_food)
• len _at_food len gets length of _at_food
• food returns index of last element

16
foreach
Visit each item in turn and call it
morsel foreach morsel (_at_food) print
"morsel\n" print "Yum yum\n"
17
Tests

• "Zero" is false. This includes 0, '0', "0", '',
  ""
• Anything not false is true
• Use and ! for numbers, eq and ne for strings
• , , and ! are and, or, and not, respectively.

18
for loops

• for loops are just as in C or Java
• for (i 0 i lt 10 i) print
  "i\n"

19
while loops
!/usr/local/bin/perl print "Password? " a
ltSTDINgt chop a Remove the
newline at end while (a ne "fred") print
"sorry. Again? " a ltSTDINgt chop
a
20
do..while and do..until loops
!/usr/local/bin/perl do print
"Password? " a ltSTDINgt chop
a while (a ne "fred")
21
if statements
if (a) print "The string is not
empty\n" else print "The string is
empty\n"
22
if - elsif statements
if (!a) print "The string is empty\n"
elsif (length(a) 1) print "The string
has one character\n" elsif (length(a) 2)
print "The string has two characters\n" else
print "The string has many characters\n"
23
Why Perl?

• Two factors make Perl important
• Pattern matching/string manipulation
• Based on regular expressions (REs)
• REs are similar in power to those in Formal
  Languages
• but have many convenience features
• Ability to execute UNIX commands
• Less useful outside a UNIX environment

24
Basic pattern matching

• sentence /the/
• True if sentence contains "the"
• sentence "The dog bites."if (sentence
  /the/) is false
• because Perl is case-sensitive
• ! is "does not contain"

25
RE special characters
. Any single character except a
newline The beginning of the line or
string The end of the line or
string Zero or more of the last
character One or more of the last
character ? Zero or one of the last
character
26
RE examples
. matches the entire string hi.bye
matches from "hi" to "bye" inclusive x y
matches x, one or more blanks, and y Dear
matches "Dear" only at beginning bags?
matches "bag" or "bags" hiss matches
"hiss", "hisss", "hissss", etc.
27
Square brackets
qjk Either q or j or k qjk
Neither q nor j nor k a-z Anything
from a to z inclusive a-z No lower
case letters a-zA-Z Any letter a-z
Any non-zero sequence of
lower case letters
28
More examples
aeiou matches one or more
vowels aeiou matches one or more
nonvowels 0-9 matches an unsigned
integer 0-9A-F matches a single hex
digit a-zA-Z matches any
letter a-zA-Z0-9_ matches identifiers
29
More special characters
\n A newline \t A tab \w Any
alphanumeric same as a-zA-Z0-9_ \W Any
non-word char same as a-zA-Z0-9_ \d Any
digit. The same as 0-9 \D Any non-digit.
The same as 0-9 \s Any whitespace
character\S Any non-whitespace character \b
A word boundary, outside only \B No
word boundary
30
Quoting special characters
\ Vertical bar \ An open square
bracket \) A closing parenthesis \
An asterisk \ A carat symbol \/ A
slash \\ A backslash
31
Alternatives and parentheses
jellycream Either jelly or cream (egle)gs
Either eggs or legs (da)
Either da or dada or
dadada or...
32
Substitution

• is a test, as in sentence /the/
• ! is the negated test, as in sentence !
  /the/
• is also used for replacement, as in
  sentence /london/London/
• This is an expression, whose value is the number
  of substitutions made (0 or 1)

33
Global substitutions

• s/london/London/
• substitutes London for the first occurrence of
  london in _
• s/london/London/g
• substitutes London for each occurrence of london
  in _
• The value of a substitution expression is the
  number of substitutions actually made

34
Case-insensitive substitutions

• s/london/London/i
• case-insensitive substitution will replace
  london, LONDON, London, LoNDoN, etc.
• You can combine global substitution with
  case-insensitive substitution
• s/london/London/gi

35
The _ variable

• Often we want to process one string repeatedly
• The _ variable holds the current string
• If a subject is omitted, _ is assumed
• Hence, the following are equivalent
• if (sentence /under/)
• _ sentence if (/under/) ...

36
Remembering patterns

• Any part of the pattern enclosed in parentheses
  is assigned to the special variables 1, 2, 3,
  , 9
• Numbers are assigned according to the left
  (opening) parentheses
• "The moon is high" /The (.) is (.)/
• Afterwards, 1 "moon" and 2 "high"

37
Dynamic matching

• During the match, an early part of the match that
  is tentatively assigned to 1, 2, etc. can be
  referred to by \1, \2, etc.
• Example
• \b.\b matches a single word
• /(\b.\b) \1/ matches repeated words
• "Now is the the time" /(\b.\b) \1/
• Afterwards, 1 "the"

38
tr

• tr does character-by-character translation
• tr returns the number of substitutions made
• sentence tr/abc/edf/
• replaces a with e, b with d, c with f
• count (sentence tr///)
• counts asterisks
• tr/a-z/A-Z/
• converts to all uppercase

39
split

• split breaks a string into parts
• info "CaineMichaelActor14, Leafy
  Drive"_at_personal split(//, info)
• _at_personal ("Caine", "Michael", "Actor", "14,
  Leafy Drive")

40
Associative arrays

• Associative arrays allow lookup by name rather
  than by index
• Associative array names begin with
• Example
• fruit ("apples", "red", "bananas", "yellow",
  "cherries", "red")
• Now, fruit"bananas" returns "yellow"
• Note braces, not parentheses

41
Associative Arrays II

• Can be converted to normal arrays_at_food
  fruit
• Cannot index an associative array, but can use
  the keys and values functionsforeach f (keys
  fruit) print ("The color of f is " .
  fruitf . "\n")

42
Associative Arrays III

• The function each gets key-value pairs
• while ((f, c) each(fruit)) print
  "f is c\n"

43
Calling subroutines

• Assume you have a subroutine printargs that just
  prints out its arguments
• Subroutine calls
• printargs("perly", "king")
• Prints "perly king"
• printargs("frog", "and", "toad")
• Prints "frog and toad"

44
Defining subroutines

• Here's the definition of printargs
• sub printargs print "_at__\n"
• Where are the parameters?
• Parameters are put in the array _at__ which has
  nothing to do with _

45
Returning a result

• The value of a subroutine is the value of the
  last expression that was evaluated

sub maximum if (_0 gt _1)
_0 else _1
biggest maximum(37, 24)
46
Local variables

• _at__ is local to the subroutine, and
• so are _0, _1, _2,
• local creates local variables

47
Example subroutine
sub inside local(a, b)
Make local variables (a,
b) (_0, _1) Assign values
a s/ //g
Strip spaces from b s/ //g
local variables (a
/b/ b /a/) Is b inside a

or a inside b? inside("lemon", "dole
money") true
48
Perl V

• Perl 5 has modules
• Perl 5 is object oriented

49
The End